Characterization of cross-linking structures in UV-cured acrylic ester resin by MALDI-MS combined with supercritical methanolysis.

The cross-linking structure of the ultra violet (UV)-cured resin prepared from dipentaerithritol hexacrylate (DPHA) was characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) combined with supercritical methanolysis. The MALDI-mass spectrum of the decomposition products obtained by supercritical methanolysis contained a series of peaks of sodium-cationized methyl acrylate (MA) oligomers up to around m/z = 4000 formed through selective cleavage and methylation occurred at ester linkages in UV-cured DPHA. Furthermore, in order to observe widely distributed sequence lengths in the cross-linking junctions, the decomposed products of the cured resin were then fractionated using size exclusion chromatography followed by the MALDI-MS measurements of the individual fractions. The MALDI-mass spectra of the lower molar mass fractions mainly consisted of a series of peaks of MA oligomers around m/z values of several thousands, whereas those of higher molecular weight showed a broad peak up to m/z ca. 180000. The observed distributions of the supercritical methanolysis products suggested that the network junctions in the given UV-cured resin were composed of up to around 2000 acrylate units.     

Dual cross-linked networks hydrogels with unique swelling behavior and high mechanical strength: based on silica nanoparticle and hydrophobic association

A series of physically cross-linked hydrogels composed poly(acrylic acid) and octylphenol polyoxyethylene acrylate with high mechanical strength are reported here with dual cross-linked networks that formed by silica nanoparticles (SNs) and hydrophobic association micro-domains (HAMDs). Acrylic acid (AA) and octylphenol polyoxyethylene acrylate with 10 ethoxyl units (OP-10-AC) as basic monomers in situ graft from the SNs surface to build poly(acrylic acid) hydrophilic backbone chains with randomly distributed OP-10-AC hydrophobic side chains. The entanglements among grafted backbone polymer chains and hydrophobic branch architecture lead to the SNs and HAMDs play the role of physical cross-links for the hydrogels network structure. The rheological behavior and polymer concentration for gelation process are measured to examine the critical gelation conditions. The correlation of the polymer dual cross-linked networks with hydrogels swelling behavior, gel-to-sol phase transition, and mechanical strength are addressed, and the results imply that the unique dual cross-linking networks contribute the hydrogels distinctive swelling behavior and excellent tensile strength. The effects of SNs content, molecular weight of polymer backbone, and temperature on hydrogels properties are studied, and the results indicate that the physical hydrogel network integrity is depended on the SNs and HAMDs concentration.     

Triple-Shape Memory Polymers Based on Self-Complementary Hydrogen Bonding

Triple shape memory polymers (TSMPs) are a growing subset of a class of smart materials known as shape memory polymers, which are capable of changing shape and stiffness in response to a stimulus. A TSMP can change shapes twice and can fix two metastable shapes in addition to its permanent shape. In this work, a novel TSMP system comprised of both permanent covalent cross-links and supramolecular hydrogen bonding cross-links has been synthesized via a one-pot method. Triple shape properties arise from the combination of the glass transition of (meth)acrylate copolymers and the dissociation of self-complementary hydrogen bonding moieties, enabling broad and independent control of both glass transition temperature (T(g)) and cross-link density. Specifically, ureidopyrimidone methacrylate and a novel monomer, ureidopyrimidone acrylate, were copolymerized with various alkyl acrylates and bisphenol A ethoxylate diacrylate. Control of T(g) from 0 to 60 °C is demonstrated: concentration of hydrogen bonding moieties is varied from 0 to 40 wt %; concentration of the diacrylate is varied from 0 to 30 wt %. Toughness ranges from 0.06 to 0.14 MPa and is found to peak near 20 wt % of the supramolecular cross-linker. A widely tunable class of amorphous triple-shape memory polymers has been developed and characterized through dynamic and quasi-static thermomechanical testing to gain insights into the dynamics of supramolecular networks.     

Rheological study of transient networks with junctions of limited multiplicity

We theoretically study the viscoelastic and thermodynamic properties of transient gels comprised of telechelic associating polymers. We extend classical theories of transient networks so that correlations among polymer chains through the network junctions are taken into account. This extension enables us to investigate how rheological quantities such as elastic modulus, viscosity, and relaxation time are affected by the association equilibrium, and how these quantities are related to the aggregation number (or multiplicity) of the junctions. In this paper, we assume, in the conventional manner, that chains are elastically effective if both their ends are connected with other chains. It is shown that the dynamic shear moduli are well described in terms of the Maxwell model. As a result of the correlation, the reduced moduli (moduli divided by the polymer concentration) increase with the concentration, but become independent of the concentration in the high-concentration range. The fraction of pairwise junctions is larger at lower concentrations, indicating the presence of concatenated chains in the system, which decreases as the concentration increases. This leads to a network relaxation time that increases with the concentration.     

Effects of monomer structure on the morphology of polymer networks and the electro-optical properties of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared by photochemical polymerisation with a series of (meth)acrylate monomers. The effects of monomer structure on the morphology of polymer networks in the PDLC films were studied. The acrylate monomers without sidegroup chain formed uniform polymer networks. The methacrylate monomers with methyl as their sidegroup chains formed lace-like networks. The size of the LC droplets increased with increasing the length of the flexible chain of both methacrylate and acrylate monomers. Meanwhile, the effects of the morphology of the polymer network on the electro-optical properties of PDLC films were also investigated.     

Simulation and theory of self-assembly and network formation in reversibly cross-linked equilibrium polymers

A simulation model of hard spheres capable of reversible assembly into chains, which then may reversibly cross-link into networks, has been studied through grand canonical Monte Carlo simulation. Effects of varying intra- and interchain bond strengths, chain flexibilities, and restrictions on cross-linking angle were investigated. Observations including chain-length distributions and phase separation could be captured in most cases using a simple model theory. The coupling of chain growth to cross-linking was shown to be highly sensitive to the treatment of cross-linking by chain ends. In some systems, ladderlike domains of several cross-links joining two chains were common, resulting from cooperativity in the cross-linking. Extended to account for this phenomenon, the model theory predicts that such cooperativity will suppress phase separation in weakly polymerizing chains and at high cross-link concentration. In the present model, cross-linking stabilizes the isotropic phase with respect to the nematic phase, causing a shift in the isotropic-nematic transition to higher monomer concentration than in simple equilibrium polymers.     

Effects of added surfactants on thermoreversible gelation of associating polymer solutions

The influence of added surfactants on physical properties of associating polymer solutions was examined by a new statistical‐mechanical theory of associating polymer solutions with multiple junctions and by computer simulation. The sol–gel transition line, the spinodal line, and the number of elastically effective chains in the mixed networks were calculated as functions of the concentration of added surfactants. All of them exhibited nonmonotonic behavior as a result of the following two competing mechanisms. One was the formation of new mixed micelles by binding surfactants onto the polymer associative groups. These micelles serve as crosslink junctions and promote gelation. The other was the replacement of polymer associative groups in the already formed network junctions by added surfactants. Such replacement lowers connectivity of junctions and destroys networks. The critical micelle concentration was also calculated. The results are compared with the reported experimental data on poly(ethylene oxide)‐based associating polymers and hydrophobically modified cellulose derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 733–751, 2004     

Real-Time NMR. How Fast Can We Do It?

Network structure development during cross-linking photopolymerization of polyethylene glycol di-acrylate and its mixture with a mono-functional 2-ethylhexyl acrylate was studied using real-time proton NMR T₂ relaxation analysis. The time resolution of the method is typically in the order of seconds. The results reveal largely heterogeneous origin of network build up at the intermediate stages of photocuring. Domains of nano-gel are already formed on initial stages of UV-curing where hardly any change in viscosity is observed. Upon increasing curing time the fraction of gel increases at the expence of sol, the molar mass of network chains decreases and the molar mass of sol increases. The presence of mono-acrylate slows down the curing rate. The curing continues after UV-illumination causing a significant increase in the amount of gel and cross-link density in the gel. Thus, the NMR method is a valuable tool for characterization of the kinetics of photopolymerization, the development of molecular structure and the resultant molecular scale heterogeneity during photocuring.     

Application of the Gaussian theory of elastomeric networks to native proteins: analysis of fluctuations and the dynamic scattering function

The junctions of an elastomeric network fluctuate about well-defined mean positions under the effects of covalently-bonded chains attached to the junctions. The residues of native proteins fluctuate about well-defined mean positions under the Lennard-Jones, dipole moment, hydrogen bond and electrostatic forces exerted by spatially neighbouring residues. Both an elastomer and a native protein are elastic bodies in this respect, and therefore share many structural features. The magnitude of fluctuations of the junctions in networks is significant, leading to their well-known interesting features. The ratio of root-mean-square fluctuations of the distance between two spatially neighbouring residues to the distance between them is of the same order of magnitude as that between two network junctions joined by a network chain. Based on the analogy of the network and protein structure, the method of evaluating the fluctuations of residues and the dynamic coherent and incoherent scattering functions is described and sample calculations for two model proteins are presented.     

Influence of Reactive Diluent on UV-curing of Acrylate Terminated Hyperbranched Polymers

A Aeries of hydroxylic hyperbranched polymers were derived from 2,2-bis (methylol) propionic acid and tris (methylol) propane reacted with acrylic acid to various extents. The obtained acrylated hyperbranched polymers alone or with a monofunctional diluent, isobornylene acrylate(IBOA) were further cured by UV radiation. The cured films based on the modified polymers alone all demonstrated poor mechanical properties due to their high network densities and low moving ability of polymer chains. For the composite systems, the cured films demonstrated improved mechanical properties due to the low network densities and high chain moving ability. With more IBOA included in the systems, acrylate groups can react to a higher extent during the curing process.     

Physically cross‐linked networks of POSS‐capped poly(acrylate amide)s: Synthesis,morphologies, and shape memory behavior

In this work, we synthesized a novel organic–inorganic semitelechelic polymer from polyhedral oligomeric silsesquioxane (POSS) and poly(acrylate amide) (PAA) via reversible addition‐fragmentation chain transfer (RAFT) polymerization. The organic–inorganic semitelechelic polymers have been characterized by means of nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, and dynamic mechanical thermal analysis. It was found that capping POSS groups to the single ends of PAA chains caused a series of significant changes in the morphologies and thermomechanical properties of the polymer. The organic–inorganic semitelechelics were microphase‐separated; the POSS microdomains were formed via the POSS–POSS interactions. In a selective solvent (e.g., methanol), the organic–inorganic semitelechelics can be self‐assembled into the micelle‐like nanoobjects. Compared to plain PAA, the POSS‐capped PAAs significantly displayed improved surface hydrophobicity as evidenced by the measurements of static contact angles and surface atomic force microscopy. More importantly, the organic–inorganic semitelechelics displayed typical shape memory properties, which was in marked contrast to plain PAA. The shape memory behavior is attributable to the formation of the physically cross‐linked networks from the combination of the POSS–POSS interactions with the intermolecular hydrogen‐bonding interactions in the organic–inorganic semitelechelics. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 587–600     

Dielectric relaxation of polymer networks built from macromolecules with dipole moment directed along the end-to-end chain vector

The dielectric relaxation properties are considered for polymer networks built from polar macromolecules with the dipole moment directed along the end-to-end chain vector. The viscoeleastic cubic model of a regular network is used. The fixed average volume of a polymer network is ensured by the effective internal pressure. The dynamic models of polymer networks with external and interchain friction are studied. Two cases are considered: (1) polar chains cross-linked in a network at their ends, and (2) a densely cross-linked network with many network junctions per polar chain. The expressions for the autocorrelation functions of the total dipole moment of a network, which determine the dielectric susceptibility, are calculated. The relaxation spectrum of the autocorrelation function consists of two regions: the high-frequency relaxation spectrum of a chain fragment between two neighbouring junctions (intrachain relaxation spectrum) and the lowfrequency interchain relaxation spectrum. The interchain relaxation spectrum is determined by cooperative motions of chains which form a network. The characteristic time of this spectrum for networks of type (1) is the relaxation time of a chain between junctions τ_min. For networks of type (2) a second time scale τ₁ exists, which corresponds to motions inside the volume occupied by a single long polar chain included in a network. It leads to different time behaviour of the autocorrelation functions for both network models. The existence of only interchain friction in the network model leads to a cut-off of the relaxation spectrum at the time τ_max depending on the volume of viscous interchain interactions.     

Synthesis of Hydrophobically Modified Poly（acrylic acid） gels and Interaction of the gels with Cationic／Anionic Surfactants

Poly(acrylic acid)(PAA) gel network with only chemical crosslinking and hydorophobically modified PAA(HM-PAA)gels with both chemical and physical crosslinking were synthesized by radical polymerization in tert-butanol,using ethylene glycol dimethacrylate (EGDMA) as crossliker,and 2-(N-ethylperfluorooctanesulfoamido)ethyl methacrylate (FMA),stearyl acrylate (SA) or lauryl acrylate (LA) as Hydrophobic comonomer respectively.The effcet of the fractions and the species of the hydrophobes on swelling properties of HM-PAA gels and the interaction of gels and surfactants were studied.The results showed that the swelling ratio of HM-PAA gels exhibited a sharp decrease with increasing hydrophobic comomomer comcentration,Which Could be sacribed to the formation of strong hydrophobic association among hydrophobic groups.It was proved that two kinds of binding mechanisms of surfactan/gel and different kinds of hydrophobic clusters existed in gels containing both physical and chemical networks.     

Concept of hard clusters in the interpretation of thermal and mechanical properties of polyurethane and polyurethane acrylate networks

Polyurethane (PU) and polyurethane acrylate (PUA) networks based on hydroxyl-terminated polycaprolactone (PCL), 1,3-bis-2,2′(2-isocyanatopropyl)benzene (m-TMXDI), trimethylolpropane (TMP) for PU or hydroxyethyl methacrylate (HEMA) for PUA were synthesized. Glass transition temperature, T_g, dynamic mechanical relaxation, α, and equilibrium tensile modulus, E′, were measured to compare the two kinds of networks. To explain thermal and mechanical properties of networks, the concept of hard clusters has been introduced. PU networks exhibit a single-phase structure with modulus and T_g dependent on the concentration of elastically active network chains (EANC) per unit volume calculated by considering hard crosslink clusters. The rigidity of the clusters comes from small diisocyanate and trimethylolpropane units connected by urethane bonds. They are embedded in a continuous soft phase of macrodiol urethane. Physical equivalence between several kinds of network models has been demonstrated for full conversion of isocyanate-alcohol reaction. PUA networks exhibit thermodynamically one-phase structures that become a two-phase structure for high molar mass of macrodiol when the molar fraction of isocyanate groups increases. For those networks, the calculated modulus considering clusters based on polyacrylate chains seems to be a good way to approach the experimental value of the equilibrium modulus. For the same molar ratio of OH to NCO groups the range of dynamic moduli is larger for PUA than for PU. This difference can be explained by a different concentration of crosslinks in the networks. © 1996 John Wiley & Sons, Inc.     

Lattice cluster theory of associating polymers. IV. Phase behavior of telechelic polymer solutions

The newly developed lattice cluster theory (in Paper I) for the thermodynamics of solutions of telechelic polymers is used to examine the phase behavior of these complex fluids when effective polymer-solvent interactions are unfavorable. The telechelics are modeled as linear, fully flexible, polymer chains with mono-functional stickers at the two chain ends, and these chains are assumed to self-assemble upon cooling. Phase separation is generated through the interplay of self-assembly and polymer/solvent interactions that leads to an upper critical solution temperature phase separation. The variations of the boundaries for phase stability and the critical temperature and composition are analyzed in detail as functions of the number M of united atom groups in a telechelic chain and the microscopic nearest neighbor interaction energy ε(s) driving the self-assembly. The coupling between self-assembly and unfavorable polymer/solvent interactions produces a wide variety of nontrivial patterns of phase behavior, including an enhancement of miscibility accompanying the increase of the molar mass of the telechelics under certain circumstances. Special attention is devoted to understanding this unusual trend in miscibility.     

Molecular aspects of polymer network deformation - small angle neutron scattering and NMR studies

Poly(1,4-butadiene) networks obtained by a 4-functional random cross-linking reaction over a broad range of polymer concentration were studied by small angle neutron scattering(SANS), ²H NMR and Monte Carlo(MC) simulation in the isotropic and uniaxially deformed state. The defect structure of the networks has been characterized by MC simulation of the cross-linking reaction. The anisotropy of the radius of gyration in deformed networks determined from SANS has been analyzed by the theory of Ullman. It was found that the number of active cross-links per chain is in agreement with MC and that the chain deformation follows phantom behaviour. The local orientation as measured by ²H NMR is related to the global anisotropy of the network by a MC calculation of oriented chains. The ²H NMR line shape of the deformed network is analyzed in terms of two relaxation processes arising from interior parts of the chains and from segments at chain ends. The mobility of both decrease with strain. It was found that the orientation connected to the first process shows the classical strain dependence of rubber elasticity, whereas the second exhibits a weaker dependence on strain.     

Interactions between brushlike polyacrylic acid side chains on a polyacrylate backbone in dioxane-water.

Densely grafted polyacrylic acids (d-PAAs) with overcrowded PAA side chains on the polyacrylate main chains were synthesized and characterized. Acryloyl poly(tert-butyl acrylate) macromonomer [M-P(tert-BA)] was prepared with a definite chain length (n=29) by atom-transfer radical polymerization (ATRP), then homopolymerization was carried out to produce densely grafted P(tert-BA)s with polyacrylate main chains of two different lengths (m=27 and 161). The two d-PAAs were obtained by hydrolyzing d-P(tert-BA)s in the presence of trifluoroacetic acid (TFA). The d-PAAs exhibit intermolecular and intramolecular hydrogen bonding between the carboxylic groups of PAA side chains in dioxane and pyridine; both were investigated using proton nuclear magnetic resonance (1H NMR) spectroscopy. The intermolecular hydrogen bonding was found to be dependent on polymer concentration, temperature, and water content. The intramolecular association between the PAA side chains was found to produce a contraction of the hydrodynamic volume of the d-PAA. Intermolecular hydrogen bonding produces aggregates, as demonstrated by dynamic light scattering (DLS). The clusters were found to shrink as the overall water concentration decreased, and this effect is tentatively explained by considering the gradient in chemical potential of water inside the clusters in comparison with the solvent phase.     

Radical polymerization of methyl methacrylate in the presence of isotactic poly(methyl methacrylate). VIII. Influence of template concentration

The influence of template concentration on the radical polymerization of methyl methacrylate along isotactic poly(methyl methacrylate) template was studied. The polymerizations were carried out on three template polymers with different molar masses in dimethylformamide at ?5°C. The initial polymerization rate increased linearly with template concentration until the distribution of template chain segments became homogeneous. At that critical concentration a strong increase in the polymerization rate was observed, whereas still higher template concentrations had only a slight effect on the polymerization rate. The polymerizations were stopped when the weight ratio of formed polymer and template was equal to one. The viscometrically determined molar mass of the formed polymers showed a remarkable behavior in the low template concentration region. It was obviously related to the molar mass of the template polymer and was lower than the molar mass found for blank polymerization. This decrease in molar mass was most pronounced in the case of the lowest template molar mass. It is suggested that nondegradative chain transfer occurring near a template chain end is responsible for this decrease. An increase in the molar mass occurred at the critical concentration, similarly to the change of polymerization rate. However, at still higher template concentrations, where template coils started to overlap each other, the molar mass of the formed polymers increased further. The growing chains could leap from one template chain to another and attain a greater chain length than the blank polymerizate.     

Preparation and properties of polyurethane networks based on α,ω-difunctional poly(hexafluoropropylene oxide)

Poly(hexafluoropropylene oxide), poly(HFPO), networks were prepared from functional polymers by end linking via urethane groups. The prepolymers were characterized by NMR spectroscopy and GPC. The networks were characterized by determination of the number of network chains from the shear modulus, and were snown to contain both trifunctional crosslinks and difunctional links. The properties of the networks were investigated by a range of techniques. Compared with fully-fluorinated networks formed via triazine cross-links, investigated previously, the urethane-linked networks were more readily prepared but were poorer elastomers, were less thermally stable, and were less resistant to swelling by common polar solvents. © 1995 John Wiley & Sons, Inc.     

Elastomeric properties of end-linked networks of high cross-link functionality. Accounting for possible changes in effective functionality with extent of reaction and chain-length distribution

This study reanalyzes some elastomeric properties in elongation reported for poly(dimethylsiloxane) (PDMS) networks of high cross-link functionality which had been prepared by using multifunctional siloxane oligomers to end link vinyl-terminated PDMS chains. The extent of reaction of the vinyl end groupsP _vi spanned the range of 0.40 to 0.95. These networks had elongation moduli that significantly exceeded the values predicted by the Flory-Erman theory, except at very low values ofP _vi. Trends in their stress-strain isotherms, as characterized by the Mooney-Rivlin constants 2C ₂ and the ratio 2C ₂/C₁, also appeared to be different from those predicted by theory. Neglected in such standard analyses, however, was the fact that the segments between cross-links along the junction precursor molecules can themselves act as short network chains, contributing to the modulus and giving a strongly bimodal distribution of both network chain lengths and cross-link functionalities. Of particular interest is the apparent change in functionality with extent of reaction and chain length distribution. The results thus obtained do suggest strong dependence of the observed values of the phantom modulus on the network chain-length distribution, particularly at very small values of the ratio of the length of the short chains to the long ones. Calculations based on recognition of these complications can be used to characterize more realistically the deformation of such networks. The results give much better agreement with experiment. Such behavior could be an important characteristic of elastomeric networks in general.Also, a preliminary attempt was made to bridge theory with experiment based on Kloczkowski, Mark, and Erman's recent theory of fluctuations of junctions in regular bimodal networks. The agreement between theory and experiment thus obtained is rather satisfactory and lends further support to assumptions that take into account the possibly bimodal nature of these high-functionality networks.